Soil organic carbon fractions in response to soil, environmental and agronomic factors under cover cropping systems: A global meta-analysis

Cover crops may improve soil health and increase soil carbon sequestration, thus contributing to both the adaptation to and mitigation of climate change. Despite these potential benefits, there currently lacks a global synthesis of the impacts of cover crops on soil organic carbon (SOC) fractions. We conducted a global metaanalysis of 93 peer-reviewed studies to quantify the effect of cover crops on changes in SOC fractions and the influence of environmental and management factors. Compared to bare soil management, cover crops increased SOC by 12% and increased seven SOC fractions, including microbial biomass carbon (MBC) by 33%, dissolved organic carbon (DOC) by 18%, particulate organic carbon (POC) by 15%, light-fraction organic carbon (LFOC) by 14%, permanganate oxidizable carbon (POXC) by 13%, short-term mineralizable carbon (SMC) by 10%, and mineral-associated organic carbon (MAOC) by 7%. The effect size of SOC was positively correlated with the effect sizes of MBC, POC, LFOC, and MAOC, but negatively correlated with the effect size of DOC. Soil taxonomic order and experimental duration were key factors affecting the beneficial effect of cover crops on the SOC fractions. Greater increases in SOC fractions due to cover crops were found in Entisols and Ultisols in comparison with other soil orders. The effect size of MAOC increased with experimental duration. Our study suggests that cover crops can significantly increase various SOC fractions, which likely serves as a building block for SOC sequestration and improvement of many aspects of soil health.